Quartile Deviation Calculator

About this calculator

Quartiles split a sorted data set into four equal parts: Q1 (25th percentile), the median (Q2), and Q3 (75th percentile). The interquartile range (IQR) is Q3 minus Q1 — the spread of the middle 50% of the data — and the quartile deviation (QD), also called the semi-interquartile range, is simply IQR divided by 2.

Because it only looks at the middle half of the data, QD is much less sensitive to extreme outliers than the standard deviation, which makes it a favorite in descriptive statistics for summarizing skewed distributions — income data, exam scores, or reaction times — where a few extreme values would otherwise distort a mean-based spread measure.